Project summary Bilinguals experience activation of both languages when they intend to use a one language. If multiple language alternatives are activated, how does a bilingual settle upon the intended alternative without interference from the unintended language? This problem is unique to bilinguals apart from monolinguals. Recent psycholinguistic and neuroscience approaches have argued for the existence of a cognitive control mechanism in the form of inhibition of the unintended language, and an emerging literature suggests that lifelong experience using cognitive control to mitigate language competition may generalize from the realm of language control to broader aspects of cognitive control. Yet, very little is known about precisely how language control comes to impact general cognitive function. Recent proposals implicate the role of language environment in how cognitive control is recruited for language selection. The proposed experiments investigate this potential locus of control by employing a longitudinal study of young adult bilinguals in Montreal before and after they switch the dominant language of their environment after secondary school. Montreal provides special test bed for the controlled study of bilingual populations. Children typically acquire French and English at an early age and attain high proficiency in both languages by secondary school, though they may be dominant in the use of either French or English. After secondary school some students enter programs in which the dominant language of the environment is switched from that of earlier development. The planned experiments exploit evidence from brain imaging, behavioral, and eye-tracking methodologies to assess the consequences of a switch in immersion context for brain structure, language selection, and executive function. Young adult speakers of French and English living in Montreal will undergo structural brain scans and be tested on a variety of cognitive and linguistic tasks at two time-points: (1) immediately after becoming immersed in the non-dominant at a post-secondary program and (2) one year following immersion. Control groups will consist of bilinguals who have not shifted their immersion context. Previous behavioral research shows that a shift in the dominant language of the immersion environment can have profound effects on lexical organization. However, the homogeneous bilingualism in Montreal allows for better control of variables that influence language processing and interactions (e.g., age of acquisition, country of origin, age of arrival, and proficiency level) isolating the variable of immersion context. Neuroimaging research shows that bilingual brains contain more grey-matter density in language and control regions (e.g., left inferior parietal lobe) compared to monolingual and that brain plasticity can be observed over relatively short time periods. Yet, no studies to our knowledge have explored structural reorganization longitudinally in bilingual speakers. Any effect of immersion context should generalize to bilinguals living elsewhere, but future research will be needed to confirm this and test how immersion might interact with other variables.